1. Field of the Invention
This invention is for a gel chemical composition. The gel composition is activated by moisture to absorb water and is used to protect electrical or other components contained within an enclosure from water damage. The composition has particular utility when introduced into confined areas such as instrument casings, housings for cable lines, splices or junction boxes, and the sheath of electric distribution, telecommunications, coaxial, or fiber optic cables to protect the contents or conductors contained therein from water damage for extended periods of time. The gel can be introduced into a housing, cable, or junction box prior to or during service. The gel composition is also incorporated into the cable itself, both between conductors in a bundle and/or between the bundles of conductors contained in the cable. Regardless of the use, not only does the gel prevent the entry of water, but the composition also eliminates shorts caused by water of the conductors in the cable, such as in telephone cables or splices which carry a small dc current, and restores current flow through the wire.
2. Description of the Prior Art
Communications cables such as telephone lines are made up of a multitude of pairs of conducting wires, typically copper wire, which are insulated from each other with a thin layer of a thermoplastic resin such as polyethylene or other materials and bundled by an insulating material. Bundles of pairs of conducting wires are then wrapped with a sheath of plastic, paper wrapping or other material, into a cable. Fillers such as a petroleum gel or an extended thermoplastic rubber compound (ETPR) are added to many cables inside the cable cover to fill the interstitial spaces, the objective being to exclude water from the cable and to prevent any water which does enter from migrating inside the cable.
Because telecommunications cable must be able to withstand exposure to water, the industry has promulgated certain performance standards which the cable must achieve. In particular, AT&T has issued a standard resulting from work conducted by Bell Laboratories which must be attained by all cable installed by the Bell operating companies that requires that a three foot long horizontal section of cable not pass any water for one hour when maintained under a column of water three feet high. The Bell companies are currently considering a change in that standard to require that a section of cable seven feet long withstand twelve feet of water head for twenty four hours. On information and belief, that latter standard has not been achieved to date.
It is likewise essential to prevent the exposure of many other types of conductors, electrical components or other instruments to invasive water. For instance, many microprocessor-controlled electromechanical devices include motors and switches which, along with the microprocessor, are driven by low voltage dc current. If exposed to moisture, such devices are susceptible to electrical shorts, especially at those points at which a circuit board is connected to the wiring and the points at which the wiring is connected to the switches and motors, because those points generally are not insulated. Even though such connections are usually enclosed by a housing or other protective covering, once moisture invades the housing, e.g., even by condensation of water vapor, those connections are readily accessible to the moisture.
In the case of telecommunications equipment, one method of providing protection for the conductors, whether in a splice or a cable, includes wrapping the bundle of conductors with a flexible material and injecting a liquid epoxy or urethane into the housing around the bundles which solidifies within the housing. Such a composition must be mixed on site and is typically injected by gravity flow into the housing. The material typically does not fill the entire interior of the casing and leaves voids. These voids or channels can create an avenue for the entrance of water, particularly at either end of the bundle that exits the housing and forms one of the wire bundles of the cable splice. The material, along with the wrapping around the splices in the housing, also can cause a funnel effect such that the water enters through a fissure in the cable sheath at either end of the closure around the cable and into the cable to the spliced wires.
As noted above, protection of the cable against water invasion is provided by filling the spaces between the wrapped bundles of conducting wires inside the cable and the interstices between pairs of conductors in each bundle (referred to as the filling zone) with compounds such as urethanes, epoxies, polystyrene foams, ETPRs, petroleum jellies and/or other hydrophobic materials. The patent literature also describes cables including water swellable polymers such as polyvinyl alcohol, polyacrylamides, or cellulose derivatives, which are applied to bundle wrappings or contained in "moisture barriers" which are spaced along the length of the cable outside of the conductor bundles and under the sheath (that area outside the bundles and under the sheath being referred to as the flooding zone).
Such cables are, however, characterized by a number of limitations and disadvantages. In the case of those which include a polymer which swells in the presence of water, such as a cellulose derivative, such polymers are typically provided in a granular or powder form. As such, distribution of the polymer throughout the flooding zone is problematical such that effective water absorbence is not assured throughout the flooding zone. Further, when sufficient water and polymer are present, the swelling of the polymer becomes problematical in a confined space. Another problem is that the water-absorbent polymer, especially in the case of cellulose derivatives and other naturally-occurring polymers, may be susceptible to bacterial attack, resulting in production of acids and other by-products which degrade the components of the cable.
Petroleum gels and ETPRs are generally used as filling compounds because the water-swellable materials used in the flooding zone are conductive once they absorb water, and also in part because all known substitutes suffer from one or more disadvantages which limit their utility such that petroleum gels represent the least expensive alternative. However, petroleum gels are also characterized by certain disadvantages. For instance, petroleum gels are relatively ineffective at water blockage in a bundle of conductors which is exposed to water because they must be applied hot. The heat tends to degrade the insulation around the individual conductors and, after cooling, the gel shrinks, leaving passages for entry and migration of water. At low temperatures, the gel imparts stiffness to the cable, hampering installation. Those same temperature-related problems affect fiber optic cables. Also, the gel is difficult to remove from the conductors during splicing and terminating the cable.
Replacing the gel with a powdered filling material which reacts with water to form a gel which blocks the water is also described in the patent literature. However, such powders are also characterized by a number of disadvantages and limitations. For instance, on contact with water, and as noted above, powders alter the electrical characteristics of the conductors in the bundles in the cable by increasing conductivity to the point that thicker insulation may be required around the conductors and the bundle, thereby increasing the cost of the cable. Using lower concentrations of the polymer in the filling material compromises the water blockage capabilities of the filling material. Further, certain swelling agents such as polyvinyl alcohols and polyacrylamides do not swell quickly enough in cold water to effect proper water blockage when the bundle is only partially filled while filling the bundle completely with such agents is prohibitively expensive and causes problems with swelling in the confined space when contacted by water.
In short, in spite of a continuing and long-felt need, and in spite of the many attempts which have been made to solve these problems, there is still a need for a water resistant cable, and specifically, for a composition which can be incorporated into a cable, splice, instrument housing, or other confined space, to prevent moisture-induced electrical problems. Specifically with regard to telecommunications cables, so far as is known, no material is available or suitable for use as both flooding and filling compound. Instead, water swellable polymers, when used at all, are generally used in the flooding zone and water resistant or hydrophobic impermeable materials are generally used in the filling zone.